A “virtual router” (an abstract representation of two or more physical routers acting as a group) can be used to increase the availability of a default gateway that services a host on a subnet. At any given time, only one of the physical routers (a “first router”) is actually routing network traffic for the host; the Internet Protocol (IP) address for the first router serves as the IP address for the virtual router, and the host uses that IP address as its default gateway. If the first router is unable to satisfactorily service the host for some reason, then it can “failover” to another physical router (a “second router”) that is spanned by the virtual router.
The Virtual Router Redundancy Protocol (VRRP) is used to assign responsibility for a virtual router to a physical router. According to VRRP, one of the physical routers in the group comprising the virtual router is assigned the highest priority (e.g., a value of 255) and consequently is identified as the owner or master, while the other physical routers in the group are assigned lower priorities and identified as backups. A backup can assume the role of master if the master cannot perform to a satisfactory level. If, for example, an interface (e.g., a port) on the master router fails, then the priority of the master router may be reduced. More specifically, according to VRRP, when an interface goes down (is out-of-service), then the priority of the master router is changed to the priority associated with the out-of-service interface. If the master's priority is reduced to less than the priority of the backup router with the next highest priority, then that backup becomes the master router.